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Hi Dave,
I hope this may answer your question...: I am sure that your assumtion that the amount of C4+ is equal to (CO3)-2 is correct. However there is one a bit confusing issue in the Teertstra & Sherriff (1997) paper - the results of the analyses recalculations in Tab 3 combine information from EPMA data acquired by the authors (most of the oxides) with values of CO2 and H2O eventually coming from analyses previously published in literature that are listed in the Table 2 as well (for example the Table 3 recalculation of the first analysis ON8 is based on the analysis in the 2nd column of the Tab 2, however it includes CO2 1.11 and H2O 0.44 from Shaw 1960 listed in 1st column of the Tab 2). Consequently, the values of C in the Table 3 are not calculated based on charge balance but they are given by analyzed CO2 content and calculated in the same way as other cations. The resulting positive/negative charge is than balanced by distribution of (analyzed) H2O to H+, OH- and H2O.
As I understand, in case that CO2 and H2O contents are unknown, the C-content is estimated simply as a (EPC-F-Cl-2*S)/2 (or eventually as EPC-F-Cl-2*S if HCO3- would be considered and A-site would not be full). In such case the C-content in the ON8 analysis would be estimated to much lower values - 0.04 (or 0.09) apfu.
cheers
Martin

2017-11-02 0:57 GMT+01:00 David Kelsey <[log in to unmask]>:

Hi all,

I have a query about how to calculate the C ions in scapolite from EPMA data, when CO2 was not measured by EPMA. I’ve been following the methodology in the Teertstra & Sherriff (1997) paper in Chemical Geology (“Substitutional mechanisms, compositional trends and the end-member formula of scapolite”) and have been using the analyses presented in that paper (their Tables 2 and 3) as a test of the my calculations. I’ve been able to replicate all the cations in the M and T sites (Si, Al, Na, Ca etc) as well as Cl and S, and also replicate their calculated “EPC” (excess positive charge). However, using their methodology I cannot replicate their values for C ions. I have been assuming – incorrectly? – that for every number of (CO3)2- ‘ions’ there are the same number of C4+ ions, as there are 1 x C ions in each (CO3)2- species. If anyone could enlighten me on what I am missing I would be extremely grateful!

 

Thanks,

 

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